It is known to link multi-user systems over large distances through leased telephone lines. For example, a workstation may be linked over a long distance to a host computer through a leased line in a telephone system. The line passes through a number of switching nodes and its bandwidth may be shared by a number of different users connected to the same host or different source end systems. The data is typically transmitted between the work station and host in a bi-directional manner, in bursts. The transmissions may be supported by asynchronous mode transmission mode (ATM) equipment. The bandwidth available to a particular transmission is initially determined by the signal source performing a routine in which the transmitted data rate is progressively increased in a test routine. The system at the destination sends back to the source a signal when the received signals become corrupted due to the data rate becoming too high, and in this way the data transmission bandwidth is set to the maximum value that the path can handle. Another bandwidth allocation technique is disclosed in EP-A-0 603 099.
When a number of signal sources and destinations share the same line, an unfair allocation of bandwidth can occur in which the shortest path tends to grab the most bandwidth because the acknowledgement signal returned during test routine occurs more quickly over a short path, so that the shorter path can acquire bandwidth more quickly. As a result, long paths tend to operate at a slower data rate than short paths. Another disadvantage is that the customer has to lease the telephone lines on a continuous basis, whereas the actual utilisation of the line varies greatly with time. For example, at night, a low utilisation rate may be achieved so that the customer has to pay for time when the line is not actually being used.
Recently, proposals have been made to integrate leased lines into a telephone network system so that the available bit rate (ABR) of the leased line can be used for other purposes during periods of low data transmission so as to permit data users to be charged on the basis of time that data is actually transmitted rather than on a flat rate leasing basis.
One proposal is to use the so-called virtual source/destination (VS/VD) configuration in which a source end system (SES) is connected to a public telephone network through a node which acts as a so-called virtual source (VS) that feeds data from the SES into the network for onward transmission to a destination end system (DES) for example a host computer or another workstation. The DES is connected to the public network through a so-called virtual destination (VD) that receives signals from the virtual source. Also, the DES can communicate back to the SES so that the virtual source and virtual destination operate bi-directionally and each comprise a VS/VD. In PCT GB 95/00502 and EP 94301673.3 a VS/VD system is described in which the bandwidth allocated to a bi-directional ATM transmission path through a public telephone network is controlled by means of a dynamic bandwidth controller (DBC). The DBC acts as a VS/VD and a connection admission control function (CAC) reviews the available bandwidth of switching nodes disposed along the transmission path so that the DBC can optimally allocate bandwidth to the path depending on operational conditions in the public network.
The DBC communicates with the SES using a specially configured form of digital ATM signal known as a resource management (RM) signal, which is used to instruct the SES to transmit at a data rate corresponding to a particular bandwidth determined by the DBC to be acceptable for the network. Also, the DBC reviews data signals received from the SES to police the data transmission rate to ensure that it falls within the acceptable bandwidth. The DBC also buffers signals received from the SES and provides certain data re-shaping and rate control functions.
The use of VS/VD architecture overcomes the contention difficulties associated with prior networks but has the disadvantage that a CAC function needs to be overlaid for all the switches of the public telephone network in order to provide the DBC with bandwidth control information. However, in practice, the network provider may wish to introduce VS/VD facilities progressively through the network and the introduction of a CAC and a DBC function as described in PCT GB 95/00502 and EP 94301673.3 would be initially expensive as it implies that all of the switches of the system would need to be replaced by switches that can handle ABR formatted signals.